Hot pin parison injection molding technique

ABSTRACT

Parisons for use in making blow molded highly molecularly oriented plastic bottles are injection molded using a cooled cavity and a relatively hot core pin to prevent the formation of residual internal stress in or near the surface of the parison. The pin is maintained at a temperature above the glass transition temperature of the resin to reduce the formation of cracks when the parison is subsequently blow molded in the orientation temperature range.

BACKGROUND OF THE INVENTION

Blow molded plastic containers are made by forming a parison which issubsequently inflated in a blow mold cavity having the configuration ofthe container. Parisons have been formed by extrusion and by injectionmolding. One technique for forming unoriented or low orientation bottlesinvolves injection molding the parison in an injection mold having acore pin on which the parison is retained. The core pin and parison aretransferred to a blow mold where pressurized gas (air) is used toinflate the hot parison. The core pin may be maintained at a temperaturehigh enough to insure that the parison remains in a sufficiently softstate for easy inflation. This technique is not economically appropriatefor blow molding highly molecularly oriented containers because theparison must be cooled to the orientation temperature range before thebottle can be blown. Another technique involves injection moldingparisons in a mold having a cooled cavity and cooled core pin to rapidlysolidify the parison. The parison is removed from the mold and stored.Later, the cold parison is reheated and inflated into conformity with ablow mold cavity. The latter process can produce highly molecularlyoriented containers if the parison is reheated to an appropriatetemperature within the orientation temperature range for the polymerprior to the blow molding step.

Molecular orientation is accomplished by stretching certain polymericmaterials at a temperature within the orientation temperature range ofthe particular polymer. Stretching sheet or film along orthogonal axesproduces biaxial orientation. Molecularly oriented materials haveimproved physical properties including superior impact resistance,increased resistance to creep, increased stiffness, and increasedresistance to rupture when compared with the same material in anunoriented state.

Biaxial orientation of blow molded containers may be accomplished byblow molding a parison which is at a temperature within the orientationtemperature range of the polymer. A high degree of orientation resultsin high resistance to creep, but can present problems such as reducedoptical clarity, stress whitening, and cracking. The degree oforientation of the container can be measured by a standard testprocedure (ASTM D-1504) which yields data in p.s.i. which arerepresentative of the degree of orientation and are referred to as"Orientation Release Stress" (O.R.S.).

For a given polymer and end use application, there is an optimum levelof orientation as determined by orientation release stress (ORS), whichmay be below the maximum possible orientation level. For example, aproperty which deteriorates with attempts to achieve high levels oforientation is optical transparency. Many polymers crack, craze, stresswhiten, show haze or otherwise become unsightly when highly oriented.

The amount of orientation in a container blow molded from a polymericmaterial is affected by the conditions under which the material isoriented. For example, in a bottle higher levels of circumferential andaxial orientation result from increasing the amount of stretch in thecircumferential and axial directions, by increasing the stretching rate,and by decreasing the stretching temperature.

For nitrile rubber containing acrylonitrile polymeric materials (such asthose polymeric materials disclosed in U.S. Pat. No. 3,426,102 to Solakor U.S. Pat. No. 3,819,762 to Howe) an orientation release stress in thecircumferential direction in excess of 500 p.s.i., preferably on theorder of 650 p.s.i., provides adequate creep resistance for pressurizedbeverage bottles having a high container volume to weight ratio. SeeU.S. Pat. No. 3,786,221 to Silverman, and McChesney et al applicationsSer. No. 319,380, now U.S. Pat. No. 3,934,743 597,678 and Ser. No.516,110, now U.S. Pat. No. 3,984,498, all assigned to the presentassignee hereby incorporated by reference. The improvement in physicalproperties due to orientation also allows the wall thickness of thebottle to be reduced for a savings of polymeric material over thatrequired for a non-oriented bottle.

While it is known to form molecularly oriented nitrile bottles by blowmolding an injection molded parison, and such techniques have met withsome success, generally they have not been economically practical forcarbonated beverage bottles. The reason has been that if the bottle isoriented by stretching sufficiently to develop the properties requiredof containers for carbonated beverages (assuming a wall thickness thinenough to be economic), stress whitening, crazing or cracking has beenobserved to occur, making the container unsalable.

Further analysis of this phenomenon has brought the realization thatcracking, crazing or stress whitening primarily develop at or near theinner surface portion of the bottle wall. This is due to the fact thethe inside of the parison is stretched to a much higher extent,proportionally, than the outside. It has been found that the degree oforientation is not constant across the bottle wall thickness, but on thecontrary varies substantially across the wall, and at or near the innersurface portion of the wall is sufficiently high to give rise to theseproblems.

In an effort to compensate for the difference in stretch between theinside and the outside, a method of heat treatment is disclosed inMcChesney et al U.S. Pat. No. 3,934,743 for achieving a more uniformcircumferential orientation across the thickness of the bottle sidewall.This is accomplished by imparting a radial temperature gradient to anaxial zone of the sidewall of the parison prior to blow molding theparison into a bottle. The inner surface of the parison is made hotterthan the outer surface of the parison to offset the difference instretch.

SUMMARY OF THE INVENTION

Parisons suitable for blow molded, highly oriented containers areinjection molded using a die cavity for the external configuration and acoaxial core pin for the internal configuration. Production efficiencyrequires that the molding cycle time be reduced to a minimum. Themolding cycle time can be minimized if the cavity and core pin arecooled by circulating coolant to solidfy the polymer rapidly. As themolten polymer is forced into a conventional cooled cavity and cooledpin injection mold, the polymer contacts the relatively cool cavity andcore pin surfaces with a resulting increase in polymer viscosity at andnear these mold surfaces and the formation of a solidifying layer on themold surfaces. The continued addition of molten polymer to completefilling of the mold causes stresses to be developed in this solidifyinglayer at and near surfaces of the parison, which stresses are retainedin the finished parison. Polymers having high viscosity or elasticity orboth in the melt processing temperature range are susceptable to highresidual stresses in the molded article. The later deliberately inducedmolecular orientation stresses add to these mold induced residualstresses. The higher degree of stretching of the inner surface of thecontainer during blow molding coupled with the injection mold inducedresidual stresses, particularly those stresses at or near the innersurface of the parison, leads to excessive stress which may manifest asstress whitening, crazing or cracks in the container. These defects areundesirable because larger cracks may lead to failure by bursting of thecontainer and smaller cracks impair the optical quality of the containerby causing hazy or other poor appearance areas in the container wall. Asa consequence, a container can not be oriented to the extent at whichthese defects appear. A higher degree of orientation could be achievedif these defects could be avoided.

According to the present invention, stress cracks in the inner portionof the wall of an oriented, blow molded container are significantlyreduced or eliminated by modifying the manner in which the parison isinjection molded to reduce or eliminate residual orientation or stressat or near the inner surface portions of the parison. According to thepresent invention, the core pin of the injection mold is maintainedabove a temperature at which significant stress will not be retained.The die cavity can be cooled as is conventional for low cycle time sincemold-induced residual stress in the outer portions of the parison bettercan be tolerated in view of the lower degree of stretch which theexterior portions of the container wall undergo during blow molding. Thepin is maintained at a temperature well above the glass transitiontemperature (Tg) of the polymer with the result that the inner portionof the parison remains in a fluid or semi-fluid state during all of thetime during which the polymer is injected. Residual stress at or nearthe inner surface portions is minimized under such conditions.

The cooled die cavity causes rapid solidification of the outer portionof the parison to provide a rigid skin thick enough to providestructural integrity to the parison as it is removed from the moldcavity and core pin to avoid distortion of the molded parison.

In the drawings:

FIG. 1 is a view in cross-section of a parison,

FIG. 2 is a view in cross-section of a bottle blow-molded from theparison of FIG. 1,

FIG. 3 is a schematic view in cross-section of the cavity and core pinfor injection molding the parison of FIG. 1,

FIG. 4 is a schematic view in partial cross-section of the blow mold pinand temperature conditioning chamber for temperature conditioning theparison for biaxial orientation during blow molding, and

FIG. 5 is a schematic view in cross-section of the blow molding pin andcavity showing the parison partially blown into the shape of the bottleof FIG. 2.

With reference to the drawings, the parison 10 shown in FIG. 1 is aclosed end tube having a configuration appropriate for blow molding abottle. The parison 10 includes neck finish 12 portion shown in the formemployed for a crown cap bottle. The neck finish may have otherconfigurations such as threads for screw caps. The wall thickness variesalong the axial length of the parison in accordance with well knownprinciples to provide the appropriate wall thickness in the finishedbottle. To encourage more biaxial stretching, the parison can be madeshorter in the axial direction than the finished bottle is tall.

The parison 10 is formed in an injection mold shown in FIG. 3. Theinjection mold comprises an axially reciprocable core pin portion 40 anda fixed die portion 30. The die portion 30 comprises a block 32 having acavity 34 of the shape of the exterior of the parison 10. The cavity 34includes a configuration 36 for the neck finish. Heat exchange fluid iscirculated through conduits 39a, b, and c to and from channels 38 in theblock 32 surrounding the cavity 34 to rapidly cool the exterior of theinjection molded parison to between about 30° F and 100° F to impartsufficient rigidity to the parison to facilitate its removal from theinjection mold. An injection gate 35 conducts molten polymer at the meltprocessing temperature into the cavity 34 under high pressure.

The reciprocable portion 40 includes an injection mold core pin 42having an exterior shape in conformance with the interior of theparison. The pin 42 is supported coaxially of the cavity 34 by areciprocable ram structure 44. The interior of the pin 42 is hollow andincludes a central "fountain" tube 46 in communication with conduit 47afor conducting temperature controlled heat exchange fluid such as oiltoward the tip of the pin. The fluid flows back from the tip of the pinoutside tube 46 and is exhausted through conduit 47b. The exhaustedfluid may be again temperature conditioned and recirculated.

In operation, the injection mold is closed by movement of the pin 42into the cavity 34. Polymer at melt processing temperature is injectedat high pressure into the space between the pin 42 and the cavity 34.The exterior surface of the thus formed parison is cooled by the colddie cavity 34 to impart sufficient rigidity to enable the parison to beejected from the pin 42 when the mold is opened. Circulating heatexchange fluid maintains the pin 42 at the desired surface temperature.

As is shown in FIG. 4, the molded parison 10 is then transferred to atemperature conditioning station in which it is retained on atemperature conditioning pin 52 in a temperature conditioning cavity 54having heaters or heat exchange fluid conduits 56. The temperatureconditioning pin and cavity closely fit the parison for good heattransfer. The temperature conditioning pin may also include heatexchange fluid channels or heater means (not shown). The temperatures ofthe conditioning pin 52 and cavity 54 are maintained so as totemperature condition the parison to bring it into a desired temperaturegradient within the orientation temperature range to provide molecularorientation when the parison is blow molded. The temperatureconditioning pin also includes passages 57 for pressurized gas used toinflate the parison in the subsequent step. Since the interior of theparison will be stretched to a greater extent than the exterior duringblow molding, a more uniform orientation throughout the thickness of thewall of the bottle may be achieved by providing the parison with atemperature gradient such that the inside is hotter than the outside.McChesney et al U.S. Pat. No. 3,934,743 and application Ser. No.597,678, provide more specific disclosures of parison temperatureconditions appropriate for molecular orientation during blow molding.

After temperature conditioning, the parison is transferred on the pin 52to a blow mold 60, shown in FIG. 5, comprising halves 63, 65. The blowmold halves 63, 65 define a cavity 62 having the shape of the finishedbottle. Coolant may be circulated through conduit 64 to cool the blowmold cavity surfaces. Air or other pressurized gas is conveyed throughpassages 57 of pin 52 to inflate the temperature conditioned parison 10,shown partially inflated in FIG. 5. Inflation continues until thestretched parison is forced into contact with the cool walls of thecavity 62 where the polymer is rapidly cooled and rigidified. The moldhalves 63, 65 are then separated and the finished bottle removed. Sinceinflation occurs while the parison is at temperatures within theorientation temperature range and the bottle walls are biaxiallystretched, the bottle is thereby biaxially oriented.

The above described procedure involving transfer of the parison from amolding pin to a temperature conditioning station is further describedin Belivakici et al application Ser. No. 651,273, assigned to thepresent assignee, filed on even date herewith, wherein an injectionmolded parison is promptly transferred to a temperature conditioning andblow pin. Alternatively, the injection molded parison can be stored forlater reheat temperature conditioning and blow molding. A furtheralternative is to utilize the injection molding pin as a blow moldingpin by transferring the parison on the pin to a blow molding cavity withor without an intervening pause in a temperature conditioning cavity.The heat exchange fluid circulating through the pin is cycled intemperature between that appropriate for injection molding according tothis invention and a lower temperature to cool the parison to theorientation temperature range for blow molding a biaxially orientedcontainer.

Comparison of parisons injection molded with hot core pins withotherwise identical parisons molded with cold core pins reveals agreater concentration of residual stress on the inner surface of thecold pin parisons. A quantitative comparison was made by performingorientation release stress (ORS) tests according to ASTM D-1504 onsections taken from the inner portions of the wall of the parison. Acomparison of parisons was also made by measuring the degree ofbirefringence using polarized light. Higher residual stress levelsprovide higher birefringence. These two comparison techniques agreedwell with each other and demonstrated the reduction in residual stresswith increasing mold core pin temperature.

According to the invention, temperature of the pin of the injection moldis preferably the minimum temperature which produces the result ofeliminating cracks due to residual stress in the parison when thecontainer is biaxially oriented to a desired ORS during blow molding.Pin temperatures higher than that minimum tend to lengthen the cycletime because the time required to cool the parison to the desiredorientation temperature for blowing increases. If the pin temperature isallowed to approach the melt processing temperature, the parisons willtend to adhere to the pin. The lowest pin temperature which avoidscracks is usually the most desirable. For a particular polymer, parisonconfiguration, container configuration and level of orientation, theselection of the injection mold pin temperature is accomplished byadjusting the heat exchange fluid flow rate or temperature until cracksare no longer formed as the container is blow molded. The selected pintemperature can be measured by use of various temperature measuringprobes or devices, but is most easily determined using graduated meltingpoint crayons such as those sold under the name TEMPILSTICK by TempilCorporation, 132 W. 22nd Street, New York 10011, N.Y.

SPECIFIC EXAMPLES INTRODUCTION

Bottles were blown from seven different polymeric materials. For eachpolymer, parisons were injection molded at various core pintemperatures. The parisons were temperature conditioned to accomplishhigh levels of molecular orientation when blown into approximately 10fluid ounce capacity bottles of a standard shape. The bottles weighedbetween 22 and 23 gms.

The bottles made from each polymeric material were blow molded in twoorientation ranges of about 500 psi and about 650 psi ORS as measured inaccordance with ASTM D-1504. A range of injection mold core pintemperatures was employed to determine the pin temperature below whichthe bottles were susceptible to crazing, haze, stress whitening orcracking. The temperature of the pin was determined within about ± 5° Fusing TEMPILSTICK crayons. Interpolation of ORS measurements tocalculate the minimum pin temperature for a particular polymericmaterial to produce crack-free bottles may have introduced an error of ±10° F, which, when coupled with the temperature measurement error of ±5° F, yields a maximum error of ± 15° F for the minimum pin temperaturereported in Table 1.

a. Injection Molding

The polymeric material pellets were molded at a water content of about0.3%. The parisons were molded to the configuration illustrated in FIG.1 on a Lombard 75 ton reciprocating screw injection machine, Model 75-6,made by Farrel Company Division, 565 Blossom Rd., Rochester, N.Y. 14610.The screw of the machine was a "PVC" type with a compression ratio of1.45:1. The mold employed is illustrated in FIG. 3. The molten polymericmaterial was injected into the mold cavity through a gate 35. The molddie 32 was cooled to as low as 40° F at the closed end of the parison bycold water circulating through conduits 39a, b, c and channels 38. Thetemperature of the core pin 42 was controlled by hot oil circulatingthrough conduit 47a, through tube 46 and back along the outside of tube46 to conduit 47b. The total mold cycle time was held between 13 and 18seconds with actual polymer injection times between 1 and 11/2 seconds.

b. Temperature Conditioning

Within 3 to 8 seconds from the opening of the injection mold, the stillwarm parisons were transferred to a temperature conditioning and blowmold pin 52 shown in FIG. 4. Blow pin 52 closely fitted the interiors ofthe parisons to establish good heat transfer. Blow pin 52 bearing aparison 10 was placed in a closely fitting thermal conditioning cavity54. The cavity 54 had five independently temperature regulated zonesalong the longitudinal axis which zones were heated by hot oilcirculating through passages 56. The blow pin temperature was regulatedby internal electric heaters. The parisons were each temperatureconditioned for a period of time approximating the injection mold cycle(13-18 seconds). The temperatures of the cavity 54 and blow pin 52 wereregulated to yield orientation levels of about 500 to 650 psi in thesubsequent blow molding step.

c. Blow Molding

At the completion of the temperature conditioning step, the blow pin 52with the parison 10 retained thereon was transferred from thetemperature conditioning cavity 54 to a blow molding station similar tothat shown in FIG. 5. The parison was inflated by introducing air underpressure through passages 57 in the blow pin 52. The parisons began toinflate at the upper or finish end and inflation then progressed towardthe tip while the inflation air pressure was maintained between 50 and75 psi. Near the end of the blowing cycle the air pressure was raised toabout 150 psi to insure conformance of the bottle with the configurationof the blow mold cavity 52. At the end of the cycle, which requiredabout 16 seconds, the pressure was released and the bottle removed fromthe mold.

d. Reported Data

The finished bottles were subjected to a test procedure in generalaccordance with ASTM D-1504 to determine the orientation release stress(ORS) at the upper, lower and middle regions of the sidewall of thebottle. For each polymeric material the average ORS at the regions mostprone to cracking was plotted against core pin temperature. Those datapoints which represented crack-free bottles were identified and aboundary drawn between clear and cracked bottles. From this plot, theminimum injection mold pin temperature for the chosen orientation levelswas determined. The thus determined minimum injection mold pintemperature for crack-free clear bottles at each of two levels oforientation release stress is tabulated in Table I.

Table I also tabulates the sources and, where known, the approximatecompositions of various polymeric materials.

The nitrile polymers from which the above bottles were made wereexamined to determine melt flow rate, die swell, and glass transitiontemperature. The polymers had a uniform moisture content of 0.3% tominimize variations due to moisture content. The resulting data arereported in Table I.

Melt flow rate was determined according to ASTM D-1238-70, Condition F,on a Model 3504 Melt Indexer available from Monsanto Company, 920 BrownSt., Akron, Ohio 44311. Melt flow rate was measured at approximately162° F above the glass transition temperature (Tg).

Die swell is a measure of polymer melt elasticity. The more elastic thematerial, the greater the die swell. More elastic materials store moreenergy and therefore tend to display greater residual stress wheninjection molded. Die swell is the percentage increase in diameter ofthe extrudate over the extrusion orifice diameter. Die swell wasmeasured using the Melt Indexer under the same conditions as were usedin determining melt flow rate.

Die swell and melt flow rate for each polymer was measured at the sameapproximately 162° F temperature increase over the glass transitiontemperature for each polymer in an effort to rely upon the observedsimilarities of polymer properties when measured at the same temperaturein excess of the glass transition temperatures of the polymers.

Glass transition temperatures were measured by the penetration method,using a du Pont Model 943 Thermal Mechanical Analyzer attachment for adu Pont 900 Differential Thermal Analyzer obtained from the InstrumentDivision, E. I. du Pont de Nemours, Wilmington, Del. 19898. These dataare reported in Table I.

                                      TABLE I                                     __________________________________________________________________________                                                         Minimum Injection                                   Approximate Composition                                                                     Glass       Mold Core Pin            Melt Pro-                       Aery-    Transition                                                                          Die                                                                              Melt                                                                             Temperature                                                                   (° F)             cessing                                                                             Polymeric                 loni-    Temp. Swell                                                                            Flow                                                                             ORS  ORS                 Temp  Material Manufacturer                                                                              Styrene                                                                            trile                                                                             Rubber                                                                             Tg (° F)                                                                     (%)                                                                              Rate                                                                             500                                                                                650                 __________________________________________________________________________                                                              psi                 392° F                                                                       Vicobar E.I. DuPont de Nemours                                                                     20%  80% 10 PHR                                                                             210   23 1.6                                                                              330  350                       Lot 429302                                                                            Plastics Department                                                           Wilmington, Delaware                                                          19898                                                           392° F                                                                       Vicobar    "         20%  80%  3 PHR                                                                             205   18 3.3                                                                              340  350                       Lot 32902                                                               392° F                                                                       Vicobar    "         20%  80%  0 PHR                                                                             208   24 7.2                                                                              350  360                       Lot 33202                                                               360° F                                                                       Experimental                                                                          Rohm & Haas, Inc.                                                                          23%  74%  0 PHR                                                                             214   21 2.6                                                                              300  320                       Nitrile Resin                                                                         Philadelphia, Pa.                                                     Lot SW73-0626                                                                         19105                                                           360° F                                                                       Barex 210                                                                             Vistron Corporation                                                                        25%  75%  9 PHR                                                                             176   21 2.5                                                                              265  *                         Lot 309 Midland Building                                                                           (methyl                                                          Cleveland, Ohio                                                                            acrylate)                                                        44115                                                           380° F                                                                       Cycopac 900                                                                           Borg-Warner Chemicals                                                                      yes* yes*                                                                              no*  208   14 2.0                                                                              260  260                               Parkersburg, W. Va.                                                           26101                                                           395° F                                                                       Cycopac 930                                                                              "         yes* yes*                                                                              yes* 213   14 1.4                                                                              260  260                 __________________________________________________________________________     *Not known or not measured                                               

From Table I it can be observed that the minimum core pin temperature,for a chosen degree of high orientation, is considerably higher than theglass transition temperature and that higher pin temperatures arerequired for those polymers which exhibit higher elastic properties inthe melt as evidenced by higher die swell.

What is claimed is:
 1. A method for making a highly molecularly orientedblow molded container substantially free of stess whitening, crazing, orcracks from polymer material comprising the steps offorming a seamless,closed ended, tubular parison from the polymeric material in aninjection mold comprising a die having a cavity cooled below the glasstransition temperature surrounding a coaxial core pin by maintaining thecore pin at a temperature, above the glass transition temperature, atwhich the polymeric material in contact with the pin will remain in astate of at least semi-fluidity throughout injecting of pressurizedpolymeric material into the die cavity and at which residual stress inthe vicinity of the inner surface of the parison is minimized in thepolymeric material and injecting pressurized molten polymeric materialinto the die cavity, temperature conditioning the injection moldedparison to within the orientation temperature range for the polymericmaterial, and inflating the temperature conditioned parison in a blowmold cavity having the configuration of the container to molecularlyorient the polymeric material to at least 500 p.s.i. orientation releasestress in the circumferential direction and to form the material intothe configuration of the container.
 2. The method of claim 1 wherein thepolymer contains acrylonitrile and styrene or methyl acrylate and theinjection mold core pin temperature is maintained in excess of 260° F.3. The method of claim 1 wherein the injection mold die is cooled tosolidify the outside of the parison.